Apparatus for and method of frequency modulating



April 21, 1942. R D, 'LL 2,280,707

APPARATUS FOR AND METHOD OF FRQUENCY MODULATING Filed May 31, 1940 2 Sheets-Sheet l 3twentor 3g attorney April 21, 1942. R. D. KELL APPARATUS FOR AND METHOD OF FREQUENCY MODULATING Filed May 31, 1940 2 Sheets-Sheet 2 3nnentor Gttorneg a method of frequency modulating.

- cuit of tube V! is coupled to the Patented Apr. 21, 1942 APPARATUS FOR AND METHOD OF FREQUENCY MODULATING Ray D. Kell, Haddonfleld, N. 1., asslgnor to Radio Corporation of America, a corporation of Delaware Application May 31, 1940, Serial No. 337,986

9 Claims.

This invention relates to an apparatus for and More particularly the invention relates to the type of frequency modulators in which the frequency is varied as a function of the applied modulation and in which the mid-frequency is stabilized.

In connection with frequency modulated radio transmitters, it is somewhat of a problem to maintain substantially constant the mid-fre quency or the unmodulated vfrequency of the transmitter. It is one of the objects of this invention to provide means for stabilizing such mid-frequency. Another object is to provide means whereby a crystal oscillator is used to stabilize the unmodulated signal from a frequency modulated transmitter. An additional object is to provide means for the conversion of constant frequency oscillations into oscillations whose frequency is varied as a function of the applied modulation.

The invention will be described by referring to the accompanying drawings, in which Figure l is a schematic circuit diagram of one embodiment of the invention; Figures 2, 3, 4a and 4b are graphic illustrations of the wave forms of the currents which are established in the operation of the circuit of Figure 1, and Figure 5 is a simplified circuit diagram of a modification of the invention.

Referring to Fig. 1, a crystal oscillator Vi is connected to a self-biased tube V2. The output circuit of tube V2 is connected to one, V3, of a pair of combining or mixing tubes V3, V4. The input circuit of the second of these tubes V4 is connected to a signal or audio input source which is indicated as a transformer l. The anode circuits of the combining tubes are connected together and to the input circuit of a modulator tube V5.

The output circuit of the modulator tube is coupled to the grid of a limiter tube V6. The output circuit of the limiter tube is coupled to the input circuit of a tube V1. The output cirinput circuit of tube V8 which has in its output circuit a tuned circuit LC4. This circuit may b connected to a load circuit, frequency multipliers, or the like.

In the operation of the circuit, the crystal oscillator generates sinusoidal currents of a frequency determined by the crystal. These currents are applied to the input circuit of tube V! which is biased by the grid current flowing through the grid leak resistor RI. The operating parameters of the tube V2 are chosen so that the applied currents operate effectively for ap- (c1. 179-1'JL5) proximately degrees. These pulses of current discharge the condenser Cl which is charged continuously through the resistor R2 and there is thus formed in the output circuit V2, currents of saw-tooth wave form illustrated in Fig. 2.

The currents of saw-tooth wave form are applied to the first, V3, of the pair of combining tubes V3, V4 and the signal or modulation currents are applied to the input circuit of the second of the combining tubes V4. The input circuit of this tube includes a series resistor R4 and a shunt capacitor C2. The value of the resistor R4 and the capacity C2 are chosen so that the currents applied to the input circuit of V4 are inversely proportional to the frequency. This relationship of the currents is that required to convert from phase to frequency modulation.

The combined currents are applied to the modulator tube V5 which is biased to obtain a characteristic indicated in Fig. 3. The graph in question shows the modulated saw-tooth waves 3 which are applied to the tube V5. The modulator tube V5 is biased to obtain the wave form illustrated by the numeral 5 in its output circuit. It will be observed that the pulses in the output circuit of tube V5 are of variable amplitude and duration; the variations corresponding to the modulation currents. The currents of saw-tooth wave of variable amplitude and duration are applied to tube V6 which cuts oil the peaks and converts the output currents to constant amplitude, as shown in Fig. 4a. It will be observed that the currents of square wave form shown in Fig. 4a have an increasing or upwardly extending characteristic at regular time intervals but a decreasing or downwardly extending characteristic which occurs at variable time intervals. .The variations correspond to the modulation component in the currents of square wave form. Currents are applied to tube V! through a capacitor resistor network C3, R5 by means of which the derivative of the currents of square wave form is derived. The nature of the wave form of the derived currents is indicated in Fig. 4b. It will be observed that the downward pulses are not only of uniform amplitude but are uniformly spaced and thus correspond to the upward per-'- tions of Fig. 4a, whereas the downward portions of the graph of Fig. 4a correspond to the rising portions of Fig. 4b. These last named pulses are of a positive character of uniform, amplitude and of variable spacing. Since the tube V! is arranged to operate only on positive pulses, the negative portions represented in Fig. 4b have no effect on the tube Vl while the positive pulses 2 V V V operate the, tube V! and are applied to the tube V8. Pulses applied to the tube V8 are resonated in the tuned circuit LCl which is adjusted to revspond to either the crystal frequency or a harmonic thereof. I

Thus in the arrangement shown, the position tuned circuit LC not only filters out the undesired or transient effects but it also produces currents, of substantially sinusoidal wave form, which have a frequency shift corresponding to the modulation currents. During the intervals when no modulation currents are applied, the resonant circuit operates at the mid-frequency or a harmonic thereof. It should be understood that the phase swing, corresponding to 300 degrees in the circuit LCI, may be increased to any desired swing by multiplying the frequency.

It will be observed that in the circuit of Fig. 1 each of theseveral tubes performs a separate operation. It shouldbe understood that two or more operations may be performed in a single tube, for example in Fig. 5, while the crystal oscillator tube V9 and the saw-tooth generator tube VIII each perform only one function, the

to derive currents of square wave form, diflerentiating said currents of square wave form to derive a series of pulses having a spacing corresponding to said modulation. and applying last mentioned pulses to obtain waves of substantially constant mid-frequency and an extreme frequency corresponding to the applied modulation.

2. The method of frequency modulating which includes generating frequency stabilized currents I of sinusoidal wave form, converting said sinusoidal wave form into currents of saw-tooth wave tubes VII and VIZ have combined functions in upward and downward swings of the combined currents of saw-tooth and modulation wave form. The final tubeof the system, VI 3, corresponds to the tube V8 of Fig. 1. Thus, it will be seen that the two tubes, VII and VI 2, perform the V which the pair 'of tubes act as limiters for both functions of tubes V3, V4, V5, V6 and V1 of Fig. 1. been described and claimed in an application Serial No. 275,151, filed May 23, 1939, by Murray Crosby and assigned to the same assignee as tion currents are combined with the saw-tooth currents. The combined currents are applied to a modulator, which selects positive pulses of variable amplitude and variable width. The positive pulses are amplitude limited to form square waves from which the derivatives are selected. The positive pulses are selected and applied to a resonant circuit. The resonant circuit converts the applied pulses into oscillatory currents of substantially sinusoidal wave form. The method may be performed by combining several of the steps within a single stage. If the saw-tooth waves of constant frequency are obtainable directly, the crystal oscillator may be omitted. The final output currents may be frequency multiplied -to obtain the desired frequency swing.

I claim as my invention:

1. The method of frequency modulating which includes generating frequency stabilized waves, applying a portion of said waves to establish waves of saw-tooth form, combining said waves of saw-tooth form with waves corresponding to the desired modulation, applying said combined waves to derive therefrom pulses having an amplitude and a width corresponding to the modulation, limiting the amplitude of said impulses This limiter-amplifier arrangement has form, combining said saw-tooth currents with modulation currents, deriving from said combined currents pulses of current of variable amplitude and variable width, converting said last named currents into currents of square wave form, differentiating said currents of square wave form, selecting from said differentiated currents pulses having spacings varying as a function of the said modulation currents, and converting said last mentioned pulses into substantially sinusoidal currents varying in frequency as a function of said modulation and having a mid-frequency corresponding to said stabilized frequency.

-' 3. The method of frequency modulating which includes deriving currents of saw-tooth wave form from currents of stable frequency characteristics, combining said saw-tooth currents with modulation currents, deriving from said combined currents pulses of current of variable amplitude and variable width, converting said last named currents into currents of square wave form, differentiating said currents of square wave form, selecting from said differentiated currents pulses having spacings varying as a function of the said modulation currents, and converting said last mentioned pulses into currentsvarying in frequency as a function of saidmodulation and having a mid-frequency corresponding to said stabilized frequency.

4. The method of frequency modulating which includes deriving currents of saw-tooth wave form from currents of stabilized frequency, combining said saw-tooth currents with modulation currents, deriving from said combined currents pulses of current of variable amplitude and variable width, limiting the amplitude of said currents of variable amplitude to form currents of saw-tooth wave form, differentiating said square wave form currents to obtain variably spaced pulses, and converting said variably spaced pulses into substantially sinusoidal currents varying in frequency as a function of the frequency of the modulation currents and having a mid-frequency corresponding to said stabilized frequency.

5. The method of frequency modulating which includes deriving currents of saw-tooth wave pulses and fixedly spaced pulses, and converting said variably spaced pulses into substantially sinusoidal currents varying in frequency as a function of the frequency of the modulation currents and having a mid-frequency corresponding to said stabilized frequency.

v 6. A device for frequency modulating including in combination a source of currents of stabilized frequency, a saw-tooth wave generator connected to said source, a source of modulating currents, means connected to said generator and said modulating current source for combining said saw-tooth and modulating currents, means connected to said combining means for converting the combined currents into currents of square wave form, means connected to said converting means for differentiating said currents to obtain variably spaced pulses and fixedly spaced pulses, and means connected to the differentiating means for converting said variably spaced pulses into 'currents of substantially sinusoidal wave form having a frequency variation corresponding to said modulating currents and a mid-frequency corresponding to said stabilized frequency.

'1. A device for frequency modulating including in combination a piezo crystal oscillator for generatingcurrents of stabilized frequency, a sawtooth wave generator connected to said oscillator, a source of-modulating currents, means connected to said saw-tooth generator and said modulating current source for combining said saw-tooth and modulating currents, a limiter connected to said combining means for converting the combined currents into currents of square wave form, means connected to said limiter for differentiating said currents to obtain variably spaced pulses and fixedly spaced pulses, and a resonant circuit connected to the differentiating means for converting said variably spaced pulses into currents of substantially sinusoidal wave form having a frequency variation corresponding to said modulating currents and a mid-frequency corresponding to said stabilized frequency.

8. A frequency modulation system including in combination a source of frequency stabilized currents of saw-tooth wave form, a modulating current source, means connected to said sources for combining said saw-tooth wave currents and said modulating current. a modulator effectively connected to said combining means for converting said combined currents into pulse currents of variable amplitude and variable duration, means connected to said converting means for limiting said variable amplitude pulses to obtain currents of square wave form, differentiating means effectively connected to said converting means for obtaining from said currents of square wave form positive pulses and negative pulses corresponding to the beginning and end of a current of square wave form respectively,and a thermionic tube in? cluding a resonant circuit effectively connected to said differentiating means so that said positive pulses acting through said tube are resonated in said resonant circuit across which frequency modulated currents may be derived. I

9. A frequency modulation system including in combination a frequency stable oscillator, a sawtooth wave generator connected to said oscillator for generating frequency stabilized currents of saw-tooth form, a modulating current source, means connected to said sources for combining said saw-tooth wave currents and said modulating current, a modulator effectively connected to said combining means for converting said combined currents into pulse currents of variable amplitude and variable duration, means connected to said converting means for limiting said variable amplitude pulses to obtain currents of square wave form, differentiating means effectivelyconnected to said converting means for obtaining from said currents of square wave form positive pulses and negative pulses corresponding to the beginning and end of a current of square wave form respectively, and a thermionic tube including a resonant circuit effectively connected to said differentiating means so that said positivepulses acting through said tube are resonated in said resonant circuit across which frequency modulated currents may be derived,

RAY I). KELL. 

